Method and apparatus for effecting handoff between different cellular communications systems

ABSTRACT

A method of effecting handoff of a mobile station from a first base station in a first cellular communications system controlled by a first mobile switching control station to a second base station in a second, different cellular system controlled by a second mobile switching control station is described. The method comprises measuring at the mobile station a parameter of a signal transmitted by said first base station and a parameter of a signal transmitted by said second base station. When the parameters reach a predetermined condition, a signal quality message is communicated from the mobile station via the first base station to said first mobile switching control station, which responds by generating information for a channel request message for the second mobile switching control station and transmitting the same to the mobile station. The mobile station generates from the information a channel request message for the second mobile switching control station and transmits the same to the second mobile switching control station. The second mobile switching control station generates channel information identifying a channel in the second communications system for the mobile station so that the handoff may be effected.

RELATED APPLICATIONS

This application is a continuation application and claims priority toU.S. application Ser. No. 10/077,094, filed on Feb. 14, 2002, andentitled “Method and Apparatus for Effecting Handoff Between DifferentCellular,” now allowed, and to U.S. Provisional Patent Application Ser.No. 60/340,242, filed Dec. 7, 2001, entitled “Method and Apparatus forEffecting Handoff Between Different Cellular Communications Systems,”which are hereby expressly incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates generally to a method of and apparatus foreffecting handoff between different cellular communications systems.

DESCRIPTION OF THE RELATED ART

The so-called code division multiple access (CDMA) modulation techniqueis but one of several techniques for facilitating communications inwhich a large number of system users are present. Although othertechniques, such as time division multiple access (TDMA), frequencydivision multiple access (FDMA) and AM modulation schemes such asamplitude companded single sideband (ACSSB) are also available, CDMA hassignificant advantages over these other modulation techniques. The useof CDMA techniques in a multiple access communication system isdisclosed in U.S. Pat. No. 4,901,307, entitled “Spread Spectrum MultipleAccess Communication System Using Satellite Or Terrestrial Repeaters”,assigned to the present assignee, the disclosure of which isincorporated herein by reference.

In U.S. Pat. No 4,901,307 a multiple access technique is described inwhich a large number of mobile telephone system users, each having atransceiver, communicate through satellite repeaters or terrestrial basestations (also known as cell base stations, or cell-sites) using codedivision multiple access (CDMA) spread spectrum communication signals.In using CDMA communications, the frequency spectrum can be reusedmultiple times thus permitting an increase in system user capacity. Theuse of CDMA techniques results in a much higher spectral efficiency thancan be achieved using other multiple access techniques.

In conventional cellular telephone systems the available frequency bandis divided into channels typically 30 KHz in bandwidth while analog FMmodulation techniques are used. The system service area is dividedgeographically into cells of varying size. The available frequencychannels are divided into sets with each set usually containing an equalnumber of channels. The frequency sets are assigned to cells in such away as to minimize the possibility of co-channel interference. Forexample, consider a system in which there are seven frequency sets andthe cells are equal size hexagons. A frequency set used in one cell willnot be used in the six nearest or surrounding neighbors of that cell.Furthermore, the frequency set in one cell will not be used in thetwelve next nearest neighbors of that cell.

In conventional cellular systems, the handoff scheme implemented isintended to allow a call or other type of connection (i.e., data link)to continue when a mobile station crosses the boundary between twocells. The handoff from one cell to another is initiated when thereceiver in the cell base station handling the call or connectionnotices that the received signal strength from the mobile station fallsbelow a predetermined threshold value. A low signal strength indicationimplies that the mobile station must be near the cell border. When thesignal level falls below the predetermined threshold value, the basestation asks the system controller to determine whether a neighboringbase station receives the mobile station signal with better signalstrength than the current base station.

The system controller in response to the current base station inquirysends messages to the neighboring base stations with a handoff request.The base stations neighboring the current base station employ specialscanning receivers which look for the signal from the mobile station onthe specified channel. Should one of the neighboring base stationsreport an adequate signal level to the system controller, then a handoffwill be attempted.

Handoff is then initiated when an idle channel from the channel set usedin the new base station is selected. A control message is sent to themobile station commanding it to switch from the current channel to thenew channel. At the same time, the system controller switches the callfrom the first base station to the second base station.

In the conventional system a call will be discontinued if the handoff tothe new base station is unsuccessful. There are many reasons that afailure in handoff may occur. Handoff can fail if there is no idlechannel available in the neighboring cell for communicating the call.Handoff can also fail if another base station reports hearing the mobilestation in question, when in fact this base station actually hears adifferent mobile station using the same channel in a completelydifferent cell. This reporting error will result in the call beingswitched to a wrong cell, typically one in which signal strength isinsufficient to maintain communications. Furthermore should the mobilestation fail to hear the command to switch channels, the handoff willfail. Actual operating experience indicates that handoff failures occurfrequently which questions the reliability of the system.

Another common problem in the conventional telephone system occurs whenthe mobile station is near the border between two cells. In thissituation the signal level tends to fluctuate at both base stations.This signal level fluctuation results in a “ping-ponging” situation inwhich repeated requests are made to hand the call back and forth betweenthe two base stations. Such additional unnecessary handoff requestsincrease the possibility of the mobile station incorrectly hearing thechannel switch command or failing to hear the command at all.Furthermore, the ping-ponging situation raises the possibility that thecall will be discontinued if it is inadvertently transferred to a cellin which all channels are currently in use and thus unavailable foraccepting the handoff.

In U.S. Pat. No. 5,101,501, entitled “Method And System For Providing ASoft Handoff In Communications In A CDMA Cellular Telephone System”,assigned to the present assignee, the disclosure of which isincorporated herein by reference, a method and system are disclosed forproviding communication with the mobile station through more than onecell base station during the handoff. In this environment communicationwithin the cellular system is uninterrupted by the eventual handoff fromthe base station corresponding to the cell from which the mobile stationis exiting to the base station corresponding to the cell to which themobile station is entering. This type of handoff may be considered as a“soft” handoff in communications between cell base stations with themobile wherein two or more base station or sectors of base stationtransmit concurrently to the mobile station. The use of such “soft”handoff techniques has been found to substantially reduce the incidenceof ping-ponging situations in which repeated handoff requests are madebetween a pair of base stations.

An improved soft handoff technique is disclosed within U.S. Pat. No.5,267,261, entitled “Mobile Station Assisted Soft Handoff In A CDMACellular Communications System”, assigned to the present assignee, thedisclosure of which is incorporated herein by reference. The softhandoff technique is improved by measuring at the mobile station thestrength of “pilot” signals transmitted by each base station within thesystem. These pilot strength measurements are of assistance in the softhandoff process by facilitating identification of viable base stationhandoff candidates.

The improved soft handoff technique prescribes that the mobile stationmonitors the signal strength of pilots from neighboring base stations.When the measured signal strength exceeds a given threshold, the mobilestation sends a signal strength message to a system controller via thebase station through which the mobile station is communicating. Commandmessages from the system controller to a new base station and to themobile station establish contemporaneous communication through the newand current base stations. When the mobile station detects that signalstrength of a pilot corresponding to at least one of the base stationsthrough which the mobile station is communicating has fallen below apredetermined level, the mobile station reports the measured signalstrength indicative of the corresponding base station to the systemcontroller via the base stations through which it is communicating.Command messages from the system controller to the identified basestation and the mobile station terminates communication through thecorresponding base station while communications through the other basestation or base stations continue.

Although the foregoing techniques are well suited to call transfersbetween cells in the same cellular system, a more difficult situation ispresented by movement of the mobile station into a cell serviced by abase station from another cellular system. One complicating factor insuch “intersystem” handoffs is that the neighboring cellular systemoften has dissimilar characteristics. For example, adjacent cellularsystems will often operate at different frequencies, and may maintaindifferent levels of base station output power or pilot strength. Thesedifferences effectively preclude the mobile station from performing thepilot strength comparisons and the like contemplated by existingmobile-assisted soft handoff techniques.

When resources are not available to conduct soft intersystem handoffs,the timing of the handoff of a call or connection from one system toanother becomes critical if uninterrupted service is to be maintained.That is, the intersystem handoff must be executed at the time mostlikely to result in successful transfer of the call or connectionbetween systems. In such a handoff, referred to herein as a hardhandoff, communication between the mobile station and one system mustcease before communication between the mobile station and the othersystem can begin. It follows that the handoff should be attempted onlywhen, for example:

-   -   (i) an idle channel is available in the new cell,    -   (ii) the mobile station is actually within range of the new cell        base station, but before it loses contact with the current cell        base station, and    -   (iii) the mobile station is in a position at which it is assured        of receiving the command to switch channels.

Ideally, each such hard intersystem handoff will be conducted in amanner which minimizes the potential for “ping-ponging” handoff requestsbetween the base stations of different systems. However, this is madedifficult as a result of the failure of existing handoff procedures toidentify when, and through which base stations, the mobile stationshould supplied with new frequency and channel information andinstructed to transfer the existing call or connection.

These and other shortcomings of existing intersystem handoff techniquesimpair the quality of cellular communications, and may be expected tofurther degrade performance as competing cellular systems continue toproliferate. Accordingly, there is a resulting need for an intersystemhandoff technique capable of reliably directing the handoff of a call orconnection between the base stations of different cellular communicationsystems.

U.S. Pat. No. 5,697,055, entitled “Mobile Station Assisted Soft HandoffIn A CDMA Cellular Communications System”, assigned to the presentassignee, the disclosure of which is incorporated herein by reference,describes a method and system for performing an intersystem handoff ofcommunication with a mobile station between base stations of first andsecond cellular systems. At the mobile station, a quantifiable parameterof a signal transmitted by a second base station of the second system ismeasured. When the measured value of the quantifiable parameter passesthrough a first predetermined level, the mobile station communicates asignal quality message via a first base station of the first system to afirst mobile switching control station.

A channel request message is then communicated from the first mobileswitching control station to a second mobile switching control stationwithin the second system. At the second base station, a quantifiableparameter of the signal received from the mobile station is alsomeasured. The second base station establishes communication with themobile station when the measured value of the quantifiable parameterpasses through a predetermined level. Alternatively, the signal strengthof a first pilot signal transmitted by the first base station ismeasured at the mobile station. A handoff request message is then sentto the second base station when the measured signal strength of thefirst pilot signal becomes less than a second predetermined level,thereby mobile station communication to be established. The provision ofa voice link between the mobile switching control stations allows forthe forwarding of an existing connection between the first and secondcellular systems, and enables the performance of soft intersystemhandoffs.

While this arrangement works well for situations where both systems areCDMA based and therefore both capable of performing soft handoff, thereremains the problem of how to handle inter-system handoff where one ormore of the systems is unable to perform such a handoff. For example,the so-called GSM standard has no mechanism for a soft handoff. Thereis, therefore, a problem in handing off a call using the air interfacefrom a CDMA network to a GSM network. Furthermore, GSM authenticationcannot be done because the CDMA 2000 mechanisms cannot transfer the datarequired to do GSM authentication. Encryption in GSM is different thanthe encryption in CDMA 2000.

One way of dealing with this problem would be to modify GSM to enable itto effect handoff to a non-GSM system, e.g. a CDMA system. However, GSMhas been established for a long time now, relatively speaking, andoperators will be reluctant to make expensive modifications to existingequipment in order to accommodate a neighboring incompatible system. Ifnew messages are added to the air interface in support of dual-modemobile stations, then modifications must be made to support these newmessages. Plainly, this is undesirable form the perspective of theoperator.

Another problem with handing off between a CDMA system and a GSM systemis that CDMA and GSM authentication use two different methods and keys.The authentication methods in GSM and CDMA 1X are basically the same,but the keys have different sizes. CDMA 1X has additional proceduressuch as unique challenge and count methods, which respectively preventchannel hijacking and replay attacks.

SUMMARY OF THE INVENTION

The invention addresses the above-discussed problems.

According to one aspect of the invention there is provided a method ofeffecting handoff of a mobile station from a first base station in afirst cellular communications system controlled by a first mobileswitching control station to a second base station in a second,different cellular system controlled by a second mobile switchingcontrol station, the method comprising: measuring at the mobile stationa parameter of a signal transmitted by said first base station;measuring at the mobile station a parameter of a signal transmitted bysaid second base station; communicating a signal quality message fromthe mobile station via the first base station to said first mobileswitching control station, when the parameters reach a predeterminedcondition; generating at the first mobile switching control stationinformation for a channel request message for the second mobileswitching control station; communicating the information from said firstmobile switching control station to said mobile station; generating atthe mobile station from the information from the first mobile switchingcontrol station a channel request message for the second mobileswitching control station; and communicating the channel request messagefrom the mobile station to the second mobile switching control station.

According to another aspect of the invention there is provided a mobilestation comprising: a first transceiver chain operable to receive andtransmit signals with a first base station in a first cellularcommunications system; a second transceiver chain operable to receiveand transmit signals with a second base station in a second cellularcommunications system; and a controller for: measuring a parameter of asignal transmitted by said first base station; measuring a parameter ofa signal transmitted by said second base station; communicating a signalquality message from the mobile station via the first base station tosaid first cellular communications system, when the parameters reach apredetermined condition; receiving from the first base stationinformation for a channel request message for the second cellularcommunications system; generating from the information from the firstbase station a channel request message for the second cellularcommunications system; and communicating the channel request message tothe second mobile station.

The above and further features of the invention are set forth withparticularity in the appended claims and together with advantagesthereof will become clearer from consideration of the following detaileddescription of an exemplary embodiment of the invention given withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic representation of a cellular system;

FIG. 2 is a schematic representation of a boundary between two cellularsystems;

FIG. 3 is a schematic diagram of a dual mode mobile station; and

FIG. 4 is a schematic representation of data exchange in a GSM system.

FIG. 5 is a schematic diagram of a single mode mobile station.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

FIG. 1 is a schematic illustration of an exemplary cellular telephonesystem. The illustrated system may utilize any of various multipleaccess modulation techniques for facilitating communications between atypically large number of system mobile stations or mobile telephones,and the base stations. Such multiple access communication systemtechniques include: time division multiple access (TDMA), frequencydivision multiple access (FDMA), code division multiple access (CDMA),and AM modulation schemes such as amplitude companded single sideband.The spread spectrum modulation technique of CDMA, disclosed for examplein the above-referenced U.S. Pat. No. 4,901,307, has significantadvantages over other modulation techniques for multiple accesscommunication systems and is therefore preferred.

In a typical CDMA system each base station transmits a unique pilotsignal, which comprises the transmission of a “pilot carrier” upon acorresponding pilot channel. The pilot signal is an unmodulated, directsequence, spread spectrum signal transmitted at all times by each basestation using a common pseudorandom noise (PN) spreading code. The pilotsignal allows the mobile stations to obtain initial systemsynchronization, i.e. timing, in addition to providing a phase referencefor coherent demodulation and a reference for signal strengthmeasurements used in handoff determination. The pilot signal astransmitted by each base station may often be the same PN spreadingcode, but with a different code phase offset.

In the system shown in FIG. 1, a system controller and switch 10, alsoreferred to as a mobile switching center (MSC), typically includesinterface and processing circuitry (not shown) for providing systemcontrol to plural base stations 12, 14 and 16. The controller 10 alsocontrols the routing of telephone calls from the public switchedtelephone network (PSTN) to the appropriate base station fortransmission to the appropriate mobile station. The controller 10 alsocontrols the routing of calls from the mobile stations, via at least onebase station to the PSTN. The controller 10 may direct calls betweenmobile users via the appropriate base station(s) since such mobilestations do not typically communicate directly with one another.

Controller 10 may be coupled to the base stations by various means suchas dedicated telephone lines, optical fiber links or by microwavecommunication links. In FIG. 1, three such exemplary base stations, 12,14 and 16 along with an exemplary mobile station 18, which includes acellular telephone, are illustrated. Arrows 20 a and 20 b define thepossible communication link between base station 12 and mobile station18. Arrows 22 a and 22 b define the possible communication link betweenbase station 14 and mobile station 18. Similarly, arrows 24 a and 24 bdefine the possible communication link between the base station 16 andthe mobile station 18.

The base station service areas or cells are designed in geographicshapes such that the mobile station will normally be closest to one basestation. When the mobile station is idle, i.e. no calls in progress, themobile station constantly monitors the pilot signal transmissions fromeach nearby base station. As illustrated in FIG. 1 the pilot signals aretransmitted to the mobile station 18 by the base stations 12, 14 and 16upon the communication links 20 b, 22 b and 24 b, respectively. Themobile station then determines which cell it is in by comparing pilotsignal strength transmitted from these particular base stations.

In the example illustrated in FIG. 1, the mobile station 18 may beconsidered closest to base station 16. When the mobile station 18initiates a call, a control message is transmitted to the nearest basestation, here base station 16. Base station 16 upon receiving the callrequest message, signals the system controller 10 and transfers the callnumber. The system controller 10 then connects the call through the PSTNto the intended recipient.

Should a call be initiated within the PSTN, the controller 10 transmitsthe call information to all base stations in the area. The base stationsin return transmit a paging message to the intended recipient mobilestation. When the mobile station hears a page message, it responds witha control message that is transmitted to the nearest base station. Thiscontrol message signals the system controller that this particular basestation is in communication with the mobile station. The controller 10then routes the call through the nearest base station to the mobilestation.

Should the mobile station 18 move out of the coverage area of theinitial base station, i.e. base station 16, an attempt is made tocontinue the call by routing the call through another base station. Inthe handoff process there are different methods of initiating thehandoff of the call or routing through another base station.

In a base station initiated handoff method, the initial base station,base station 16, notices that the signal transmitted by the mobilestation 18 has fallen below a certain threshold level. The base station16 then transmits a handoff request to the system controller 10, whichrelays the request to all neighboring base stations 12, 14 of the basestation 16. The controller-transmitted request includes informationrelating to the channel, including the PN code sequence used by mobilestation 18. Base stations 12 and 14 tune a receiver to the channel beingused by the mobile station and measure the signal strength, typicallyusing digital techniques. If one of base stations 12 and 14 receiversreport a stronger signal than the initial base station reported signalstrength, then a handoff is made to that base station.

Alternatively, the mobile station itself may initiate a so-calledmobile-assisted handoff. The base stations each transmit a pilot signal,which, among other things, identifies the base station. The mobilestation is equipped with a search receiver that is used to scan thepilot signal transmission of the neighboring base stations 12 and 14, inaddition to performing other functions. If the pilot signal of one ofthe neighboring base stations 12 and 14 is found to be stronger than agiven threshold, then the mobile station 18 transmits a message to thiseffect to the current base station 16.

An interactive process between the mobile station and the base stationthen permits the mobile station to communicate through the one or moreof base stations 12, 14 and 16. During this process the mobile stationidentifies and measures the signal strength of the pilot signals that itreceives. This information is communicated, via the base station(s) withwhich the mobile station is communicating, through to the MSC. The MSC,upon receiving this information, initiates or terminates connectionsbetween the mobile and base stations, thereby affecting themobile-assisted handoff.

The foregoing process may also be considered to be a “soft” handoff inthat the mobile station simultaneously communicates through more thanone base station. During a soft handoff the MSC can combine or choosebetween the signals received from each base station with which themobile unit is in communication during movement between different cells.In like manner the MSC may relay signals from the PSTN to each basestation with which the mobile unit is in communication. Mobile-assistedhandoffs tend to be more complex if the mobile station happens to belocated within the coverage area of two or more base stations not withinthe same cellular system, i.e., not controlled by the same MSC.

One approach to performing a handoff between base stations withindifferent systems will now be described with reference to FIG. 2, whichshows in schematic form a cellular communications network 30 in whichare included a CDMA cellular system (e.g. IS-95 1X) under the control ofa CDMA mobile switching center MSCc and a GSM cellular systems under thecontrol of a GSM mobile switching center MSCg. In FIG. 2, there areillustratively represented five such exemplary base stations B1A to B5Arespectively located within cells C1A to C5A of the CDMA system, andfive base stations B1B to B5B respectively located within the cells C1Bto C5B of the GSM system. Although, for convenience of illustration, thecells C1A to C5A and C1B to C5B are shown as being circular, it shouldbe understood that cells will typically be designed to be of othershapes and in reality will have forms dependent on the terrain andtopography of the area in which they are located. In what follows cellsC1A to C3A and C1B to C3B may be referred to as “border” cells, sincethese cells are proximate the boundary between the first and secondcellular systems. This designation allows the remainder of the cellswithin each system to be conveniently referred to as “internal” cells.

The following description will be given with reference to a mobilestation, which is capable of receiving and reacting to signals from basestations within both CDMA and GSM cellular systems. It is contemplated,however, that any types of communication systems may be used, such asCDMA One, CDMA2000, CDMA 2000 1x, CDMA 2000 3x, High Data RatePrinciples (HDR), CDMA 1xEV, CDMA 1xEVDO, TDMA, TDSCDMA, W-CDMA, GPRSand others. To this end, in one embodiment, the mobile station isconfigured with a dual-band transceiver having a receive chain tuneableto the different operating frequencies of the two cellular systems. Aschematic diagram of such a mobile station is given in FIG. 3 of theaccompanying drawings. As shown therein the mobile station 40 comprisesan antenna 42 connected through a diplexer 44 to both a CDMAtransmission and reception chain 46 and a GSM transmission and receptionchain 48. The transmission/reception chains 46, 48 are conventional forthe respective CDMA and GSM systems. The chains output suitablydemodulated and converted data to a convention baseband circuit 50, andreceive data for transmission from the baseband circuit 50. Thetransmission/reception chains 46, 48 are controlled by a controller 52,which, among other things, switches between the two chains in responseto command signals from the CDMA or GSM system. Thus, in thisembodiment, the two chains are not active at the same time. In anotherembodiment, the two chains may be active at the same time.

In another embodiment, the mobile station is configured with a singletransceiver having a receive chain tuneable to one of the two cellularsystems. A schematic diagram of such a mobile station is given in FIG. 5of the accompanying drawings. As shown therein the mobile station 53comprises an antenna 54 connected through a diplexer 55. The diplexer isconnected to either a CDMA transmission and reception chain 56 (if it'sa CDMA handset) or a GSM transmission and reception chain 57 (if it's aGSM handset). The transmission/reception chains 56, 57 are conventionalfor their respective CDMA and GSM systems. The chain output is suitablydemodulated and converted data to a convention baseband circuit 58, andreceives data for transmission from the baseband circuit 58. Thetransmission/reception chain, either chain 56 or chain 57, is controlledby a controller 59.

Returning to FIG. 2, the CDMA mobile switching center (MSCc) controlsthe routing of telephone calls from the public switched telephonenetwork (PSTN) to the appropriate base station B1A to B5A fortransmission to the designated mobile station. The CDMA mobile switchingcenter MSCc also controls the routing of calls from the mobile stationswithin the coverage area of the first cellular system, via at least onebase station, to the PSTN. The GSM mobile switching center MSCg operatesin a like manner to govern the operation of the base stations B1B toB5B, and to route calls between the PSTN and the GSM cellular system.Control messages and the like are communicated between MSCc and MSCgover an intersystem data link 34.

When a mobile station is located within an internal cell of the CDMAsystem, the mobile station will typically be programmed to monitor thepilot signal transmissions from each nearby (i.e., internal and/orborder) base station. The mobile station then determines which internalcell it is in by comparing pilot signal strength transmitted from thesurrounding base stations. When the mobile station approaches theboundary of the internal cell, a mobile-assisted handoff may beinitiated in the manner described above with reference to U.S. Pat. No.5,267,261, for example.

A different situation exists when the mobile station is located withinone of the border cells C1A to C3A or C1B to C3B. As an example,consider a case in which the mobile station is located within cell C2A,but is approaching cell C2B. In this instance the mobile station couldbegin to receive usable signal levels from base station B2B, which wouldthen be reported to base station B2B and to any other base station(s)with which the mobile station is currently in communication. The time atwhich usable signal levels are being received by a mobile or basestation may be determined by measuring one or more quantifiableparameters (e.g., signal strength, signal to noise ratio, frame erasurerate, bit error rate, and/or relative time delay) of the receivedsignal. The mechanism is similar to that described in the aboveidentified U.S. Pat. No. 5,697,055.

If both systems were CDMA systems, then the handoff mechanism describedin U.S. Pat. No. 5,697,055 could be used effect the handoff between cellC2A and cell C2B. There is, however, a problem in that there iscurrently no mechanism for handing off a call using the air interfacefrom a CDMA network to a GSM network. GSM authentication cannot be donebecause the CDMA mechanisms cannot transfer the data required to do GSMauthentication. Encryption in GSM is different than the encryption inCDMA. If new messages are added to the air interface in support ofdual-mode mobile stations, then modifications must be made to supportthese new messages. This is undesirable.

The solution to this problem is to use a generic message containinginstructions that enable the mobile station to transfer from the CDMAnetwork to the GSM network. The generic message must be able to conveydata necessary to effect GSM authentication and encryption. Preferably,other supplementary features in GSM should also be supported by thegeneric message. In other words, established GSM protocols must be keptintact so as to minimize any changes in existing GSM systems. Part ofthe handoff operation includes establishing subscriber identity and oncethe handoff has been affected it is necessary to maintain signaling anddata confidentiality for physical connections (ciphering). Thedefinition and operational requirements of subscriber identityauthentication are given in GSM 02.09.

The authentication procedure is also used to set the ciphering key.Therefore, the authentication procedure is performed after the networkhas established the subscriber identity and before the channel isencrypted. Two network functions are necessary in order to achieve this,namely the authentication procedure itself, and management ofauthentication and encryption keys within the system.

With this in mind, the idea is to make use of tunneling mechanisms thatmay work at any time (during hand-off situations and non-hand-offsituations), and may be uni-directional or bi-directional. One type oftunneling mechanism is the so-called ADDS (Application Data DeliveryService) messages and short data burst messages to transparently passwithin the CDMA system GSM parameters that are typically not examined bythe GSM Base Station Controller BSC, but are needed by a dual modemobile station. The use of ADDS messages together with data burstsallows a generic payload to be sent between the mobile service switchingcenters (MSC) of the networks or other network elements (e.g. SMS,position location server, OTASP). The system takes advantage of this topass GSM information end-to-end between the network and the mobilestation without requiring any changes to the CDMA BSCc or BTSc.

In the network arrangement shown in FIG. 2, ADDS messages that are usedto convey GSM handoff data, such as timing information andauthentication data from the MSCc through the BSCc to the mobilestation. The mobile station then uses so-called MAP (Mobile ApplicationProtocol) messages to convey the handoff data to the MSCg in the GSMnetwork. This requires only a small change to the MSCg to enable it tointerpret the data in the MAP messages and control the mobile stationaccordingly. Other alternatives for transferring the data are, ofcourse, possible.

When the mobile station is at the border between the CDMA and GSMsystems (e.g. in cell C2A and approaching cell C2B) the mobile stationbegins the handoff process by sending a message back to the MSCcnotifying the MSCc that conditions are such that the mobile should behanded off to the GSM system.

In a GSM system two types of handoff are available, namely synchronousand asynchronous. For ease of implementation asynchronous handoff ispreferred. The mobile station is therefore told that the handoff will bean asynchronous handoff to GSM. After a handoff order is received by themobile station the mobile first sends a few access bursts to the GSMbase station controller BSCg until it receives until it receives back aMAP handoff message which is passed back to the CDMA MSCc to enable GSMauthentication data to be generated and provided to the mobile station.GSM has a procedure for asynchronous handoff, with data bursts whichhelp the BSCg to acquire timing for mobile. The ADDS message thereforeincludes an ‘action time’ message specifying a specific time for handoffto happen. Only once this data has been received will the mobile startnormal transmission.

Another problem with handing off between CDMA and GSM is that CDMA andGSM authentication use two different methods and keys. Theauthentication methods in GSM and CDMA 1X are basically the same, butthe keys have different sizes. CDMA 1X has additional procedures such asUnique Challenge and Count methods, which respectively prevent channelhijacking and replay attacks. For a CDMA physical layer to be usedwithin a GSM system without requiring significant modifications to theGSM MSCg, GSM authentication methods should be re-used over the CDMAphysical layer. This provides the advantage of the system not having tosupport two different types of authentication centers, two types of SIMcards, etc.

The authentication procedure consists of a series of exchanges betweenthe system and the mobile station. The system transmits anon-predictable number RAND to the mobile station. Next, the mobilestation computes a result SRES, also known as the signature of the RANDnumber, using an algorithm known as the A3 algorithm. The A3 algorithmuses RAND and an Individual Subscriber Authentication Key Ki tocalculate SRES. The Subscriber Authentication Key Ki is allocated whenthe customer first subscribes to the service and is stored both in a SIM(subscriber identity module) card and in the Home Location Register(HLR) of the system. Ki is the private key in the encryption andtherefore is never transmitted over the network. Finally, the mobilestation transmits the signature SRES to the system where it is testedfor validity.

FIG. 4 of the accompanying drawing illustrates how authentication iseffected in the GSM MSC. The authentication key in GSM is called Ki andis 128 bits long. The network generates a random number (RAND), which isalso 128 bits in length. RAND and Ki are input to the A3 algorithm,which calculates a 32-bit result (SRES) from the input data. The RANDnumber is also transmitted to the mobile station by way of over the airmessages. In a GSM system each mobile station includes a smart card,i.e. the so-called SIM (subscriber identity module) card. Standard SIMcommands for authentication are specified in GSM 11.11. These commandsare only allowed to be executed if they do not interfere with thecorrect functioning of the GSM application. If the SIM is removed fromthe mobile station during a call, the call is terminated immediately, asdefined in GSM 11.11.

The SIM in the mobile station also computes SRES by applying the A3algorithm to the received RAND number and a locally stored copy of Ki.The result of the computation is again SRES and should be the same asthe SRES calculated by the network. The result SRES is therefore sent bythe mobile station to the network where it is compared with the value ofSRES calculated by the network. If both values of SRES are the same thenthe mobile station is authentic. In the system of FIG. 2 the RAND numberis transmitted using the ADDS messages on the air interface and a resultSRES is transmitted back.

The value of SRES is also used in an algorithm known as A8 to calculatean 64-bit encryption or ciphering key Kc. The Kc key generated by theGSM authentication and encryption algorithms by the SIM in the mobilestation is applied to the CDMA physical layer in place of the privatelong code mask that would normally be generated using the CDMA CAVEalgorithm. The 64-bit Kc key is uniquely mapped to the 42 bit privatelong code and, thus, is used as a basis for the “private long code mask”to provide for voice privacy. The private long code mask is passedaround CDMA messages and interpreted no differently than if it had beengenerated from the CAVE algorithm. Using this approach for voice privacyallows the system to keep a unique authentication center and unique SIMtypes, within the hybrid CDMA/GSM network.

GSM performs encryption at the frame level. Every frame is encryptedusing the frame number and the 64-bit Kc key, which key is derived asdiscussed with reference to FIG. 4. The frame number and Kc mask isapplied to every frame. In the CDMA 1X system the encryption isperformed using a 42-bit private long code. In the hybrid system of FIG.2 the Kc key is used to derive a 42-bit private long code mask, with amapping algorithm mapping between Kc and the private long code. Thismapping is performed in the MSCc, which then simply tells the BSC whichprivate long code to use.

The ADDS operation allows the transfer of transparent services betweenterrestrial network elements (e.g. MSC, SMS, PDC) and the mobilestation. The system uses this operation to transfer the Authenticationinformation RAND to the MS and to transfer SRES back to the MSC. TheADDS messaging operation goes from the MSCc to the BSCc, and allows datato be sent to the mobile station over the paging channel. The ADDSTransfer operation goes from the BSCc to the MSCc and allows data to besent to the network from the mobile station over the access channel. TheADDS Deliver operation goes from the MSCc to the BSCc, or BSCc to MSCcand allows data to be sent between the mobile station and the networkover the traffic channel. An ADDS parameter has been defined as “ADDSUser Part”, which contains a 6-bit “Data Burst Type” that indicates theformat of the application data message. The ADDS operation utilizes theADDS User Part parameter to contain the service-specific data. Theauthentication operation makes use of the ADDS User Part to carry theauthentication data. The described system uses a new Data Burst Typenamed “GSM-MAP Authentication” which is interpreted accordingly by themobile station.

It should be noted that the exemplary embodiments may be implementedwhenever a database for storing information pertaining to theauthentication process exists at the receiving end, or is accessible bythe receiving end. The processor of the exemplary embodiments may beused to implement one cryptographic scheme with one party and anothercryptographic scheme with another party. The basic implementation of theexemplary embodiments may be performed without the need for physicalconnection to intermediary resources because communication with separateparties occur through a wireless medium.

Those of skill in the art would understand that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Thevarious illustrative components, blocks, modules, circuits, and stepshave been described generally in terms of their functionality. Whetherthe functionality is implemented as hardware or software depends uponthe particular application and design constraints imposed on the overallsystem. Skilled artisans recognize the interchangeability of hardwareand software under these circumstances, and how best to implement thedescribed functionality for each particular application. As examples,the various illustrative logical blocks, flowcharts, windows, and stepsdescribed in connection with the embodiments disclosed herein may beimplemented or performed in hardware or software with anapplication-specific integrated circuit (ASIC), a programmable logicdevice, discrete gate or transistor logic, discrete hardware components,such as, e.g., registers in the FIFO, a processor executing a set offirmware instructions, any conventional programmable software and aprocessor, a field programmable gate array (FPGA) or other programmablelogic device, or any combination thereof. The processor mayadvantageously be a micro-controller, but in the alternative, theprocessor may be any conventional processor, controller,micro-controller, or state machine. The software may reside in RAMmemory, flash memory, ROM memory, EPROM memory, EEPROM memory, harddisk, removable disks, a CD-ROM, a DVD-ROM, registers, or any othermagnetic or optical storage media. Those of skill of the art wouldfurther appreciate that the data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description are advantageously represented by voltages, currents,electromagnetic waves, magnetic field or particles, optical fields orparticles, or any combination thereof.

Having thus described the invention by reference to a preferredembodiment it is to be well understood that the embodiment in questionis exemplary only and that modifications and variations such as willoccur to those possessed of appropriate knowledge and skills may be madewithout departure from the spirit and scope of the invention as setforth in the appended claims and equivalents thereof.

1. A method of effecting handoff of a mobile station from a first basestation in a first cellular communications system controlled by a firstmobile switching control station to a second base station in a secondcellular system controlled by a second mobile switching control station,said first cellular communications system being a CDMA system, and saidsecond cellular system being a GSM system, the method comprising:receiving a signal quality message from the mobile station via the firstbase station at said first mobile switching control station, when afirst parameter of a first signal transmitted by the first base stationand a second parameter of a second signal transmitted by said secondbase station, as measured by the mobile station, reach a predeterminedcondition; generating at the first mobile switching control station anApplication Data Delivery Service (ADDS) message containing handoff datain response to the signal quality message, the ADDS message being a typeof tunnelling mechanism, which transparently passes the handoff datawithin the CDMA system, wherein the handoff data comprises GSMparameters including timing information for effecting the handoff to theGSM system; and communicating the ADDS message from said first mobileswitching control station to said mobile station.
 2. The method asclaimed in claim 1, further comprising: receiving, at the second mobileswitching control station from the mobile station, a Mobile ApplicationProtocol (MAP) message containing the handoff data; and generating atthe second mobile switching control station channel informationidentifying a channel in the second communications system for the mobilestation.
 3. The method as claimed in claim 2, further comprisingestablishing communication between said mobile station and said secondbase station in the identified channel.
 4. The method as claimed inclaim 3, further comprising discontinuing communication between saidmobile station and said first base station.
 5. The method as claimed inclaim 1, wherein said first and second parameters correspond to signalstrength.
 6. The method as claimed in claim 1, wherein said handoff dataincludes authentication data.
 7. A mobile station, comprising: a firsttransceiver chain operable to receive and transmit signals with a firstbase station in a first cellular communications system, said firstcellular communications system being a CDMA system; a second transceiverchain operable to receive and transmit signals with a second basestation in a second cellular communications system, said second cellularcommunications system being a GSM system; and a controller for:measuring a first parameter of a first signal transmitted by said firstbase station; measuring a second parameter of a second signaltransmitted by said second base station; communicating a signal qualitymessage from the mobile station via the first base station to said firstcellular communications system, when the first and second parametersreach a predetermined condition; receiving from the first base stationan Application Data Delivery Service (ADDS) message containing handoffdata in response to the signal quality message, the ADDS message being atype of tunnelling mechanism, which transparently passes the handoffdata within the CDMA system, wherein the handoff data comprises GSMparameters including timing information for effecting the handoff to theGSM system; generating a Mobile Application Protocol (MAP) messagecontaining the handoff data; and communicating the MAP message to thesecond base station.
 8. The mobile station as claimed in claim 7,wherein the controller is further for receiving from said second basestation channel information identifying a channel in the secondcommunications system for the mobile station.
 9. The mobile station asclaimed in claim 8, wherein the controller is arranged to respond to thechannel information by establishing communication between said mobilestation and said second base station in the identified channel.
 10. Themobile station as claimed in claim 9, wherein the controller is arrangedto respond to the channel information by discontinuing communicationbetween said mobile station and said first base station.
 11. The mobilestation as claimed in claim 7, wherein said first and second parameterscorrespond to signal strength.
 12. The mobile station as claimed inclaim 7, wherein said handoff data includes authentication data.
 13. Themobile station as claimed in claim 7, wherein the first transceiverchain is active when the second transceiver chain is inactive.
 14. Themobile station as claimed in claim 7, wherein the second transceiverchain is active when the first transceiver chain is inactive.
 15. Anapparatus for effecting handoff of a mobile station from a first basestation in a first cellular communications system controlled by a firstmobile switching control station to a second base station in a secondcellular system controlled by a second mobile switching control station,said first cellular communications system being a CDMA system, and saidsecond cellular system being a GSM system, the apparatus comprising:means for receiving a signal quality message from the mobile station viathe first base station at said first mobile switching control station,when a first parameter of a first signal transmitted by the first basestation and a second parameter of a second signal transmitted by saidsecond base station, as measured by the mobile station, reach apredetermined condition; means for generating at the first mobileswitching control station an Application Data Delivery Service (ADDS)message containing handoff data in response to the signal qualitymessage, the ADDS message being a type of tunnelling mechanism, whichtransparently passes the handoff data within the CDMA system, whereinthe handoff data comprises GSM parameters including timing informationfor effecting the handoff to the GSM system; and means for communicatingthe ADDS message from said first mobile switching control station tosaid mobile station.
 16. The apparatus as claimed in claim 15, furthercomprising: means for receiving, at the second mobile switching controlstation from the mobile station, a Mobile Application Protocol (MAP)message containing the handoff data; and means for generating at thesecond mobile switching control station channel information identifyinga channel in the second communications system for the mobile station.17. The apparatus as claimed in claim 16, further comprising means forestablishing communication between said mobile station and said secondbase station in the identified channel.
 18. The apparatus as claimed inclaim 17, further comprising means for discontinuing communicationbetween said mobile station and said first base station.
 19. Theapparatus as claimed in claim 15, wherein said first and secondparameters correspond to signal strength.
 20. The apparatus as claimedin claim 15, wherein said handoff data includes authentication data. 21.A computer program product comprising a non-transitory computer readablemedium storing a code for effecting handoff of a mobile station from afirst base station in a first cellular communications system controlledby a first mobile switching control station to a second base station ina second cellular system controlled by a second mobile switching controlstation, said first cellular communications system being a CDMA system,and said second cellular system being a GSM system, the code whenexecuted by a computer causing the computer to: receive a signal qualitymessage from the mobile station via the first base station at said firstmobile switching control station, when the a first parameter of a firstsignal transmitted by the first base station and a second parameter of asecond signal transmitted by said second base station, as measured bythe mobile station, reach a predetermined condition; generate at thefirst mobile switching control station an Application Data DeliveryService (ADDS) message containing handoff data in response to the signalquality message, the ADDS message being a type of tunnelling mechanism,which transparently passes the handoff data within the CDMA system,wherein the handoff data comprises GSM parameters including timinginformation for effecting the handoff to the GSM system; and communicatethe ADDS message from said first mobile switching control station tosaid mobile station.
 22. The computer program product as claimed inclaim 21, further comprising the code, which when executed by a computercausing the computer to: receive, at the second mobile switching controlstation from the mobile station, a Mobile Application Protocol (MAP)message containing the handoff data; and generate at the second mobileswitching control station channel information identifying a channel inthe second communications system for the mobile station.
 23. Thecomputer program product as claimed in claim 22, further comprising thecode, which when executed by a computer causing the computer to:establish communication between said mobile station and said second basestation in the identified channel.
 24. The computer program product asclaimed in claim 23, further comprising the code, which when executed bya computer causing the computer to: discontinue communication betweensaid mobile station and said first base station.
 25. The computerprogram product as claimed in claim 21, wherein said first and secondparameters correspond to signal strength.
 26. The computer programproduct as claimed in claim 21, wherein said handoff data includesauthentication data.
 27. A computer program product comprising anon-transitory computer readable medium storing a code for effectinghandoff of a mobile station, the code when executed by a computercausing the computer to: receive and transmit signals with a first basestation in a first cellular communications system, said first cellularcommunications system being a CDMA system; receive and transmit signalswith a second base station in a second cellular communications system,said second cellular communications system being a GSM system; measure afirst parameter of a first signal transmitted by said first basestation; measure a second parameter of a second signal transmitted bysaid second base station; communicate a signal quality message from thecomputer via the first base station to said first cellularcommunications system, when the first and second parameters reach apredetermined condition; receive from the first base station anApplication Data Delivery Service (ADDS) message containing handoff datain response to the signal quality message, the ADDS message being a typeof tunnelling mechanism, which transparently passes the handoff datawithin the CDMA system, wherein the handoff data comprises GSMparameters including timing information for effecting the handoff to theGSM system; generate a Mobile Application Protocol (MAP) messagecontaining the handoff data; and communicate the MAP message to thesecond base station.
 28. The computer program product as claimed inclaim 27, wherein said first and second parameters correspond to signalstrength.
 29. At least one processor for effecting handoff of a mobilestation, comprising: a first processor component to receive and transmitsignals with a first base station in a first cellular communicationssystem, said first cellular communications system being a CDMA system,wherein the first processor component comprises hardware; a secondprocessor component to receive and transmit signals with a second basestation in a second cellular communications system, said second cellularcommunications system being a GSM system; a third processor component tomeasure a first parameter of a first signal transmitted by said firstbase station; a fourth processor component to measure a second parameterof a second signal transmitted by said second base station; a fifthprocessor component to communicate a signal quality message from thecomputer via the first base station to said first cellularcommunications system, when the first and second parameters reach apredetermined condition; a sixth processor component to receive from thefirst base station an Application Data Delivery Service (ADDS) messagecontaining handoff data in response to the signal quality message, theADDS message being a type of tunnelling mechanism, which transparentlypasses the handoff data within the CDMA system, wherein the handoff datacomprises GSM parameters including timing information for effecting thehandoff to the GSM system; a seventh processor component to generate aMobile Application Protocol (MAP) message containing the handoff data;and an eighth processor component to communicate the MAP message to thesecond base station.
 30. The at least one processor as claimed in claim29, wherein said first and second parameters correspond to signalstrength.
 31. A mobile station, comprising: means for receiving andtransmitting signals with a first base station in a first cellularcommunications system, said first cellular communications system being aCDMA system; means for receiving and transmitting signals with a secondbase station in a second cellular communications system, said secondcellular communications system being a GSM system; means for measuring afirst parameter of a first signal transmitted by said first basestation; means for measuring a second parameter of a second signaltransmitted by said second base station; means for communicating asignal quality message from the computer via the first base station tosaid first cellular communications system, when the first and secondparameters reach a predetermined condition; means for receiving from thefirst base station an Application Data Delivery Service (ADDS) messagecontaining handoff data in response to the signal quality message, theADDS message being a type of tunnelling mechanism, which transparentlypasses the handoff data within the CDMA system, wherein the handoff datacomprises GSM parameters including timing information for effecting thehandoff to the GSM system; means for generating a Mobile ApplicationProtocol (MAP) message containing the handoff data; and means forcommunicating the MAP message to the second base station.
 32. The mobiledevice as claimed in claim 31, wherein said first and second parameterscorrespond to signal strength.
 33. A method of effecting handoff of amobile station from a first base station in a first cellularcommunications system controlled by a first mobile switching controlstation to a second base station in a second cellular system controlledby a second mobile switching control station, said first cellularcommunications system being a CDMA system, and said second cellularsystem being a GSM system, the method comprising: measuring a firstparameter of a first signal transmitted by said first base station;measuring a second parameter of a second signal transmitted by saidsecond base station; communicating a signal quality message from themobile station via the first base station to said first cellularcommunications system, when the first and second parameters reach apredetermined condition; receiving from the first base station anApplication Data Delivery Service (ADDS) message containing handoff datain response to the signal quality message, the ADDS message being a typeof tunnelling mechanism, which transparently passes the handoff datawithin the CDMA system, wherein the handoff data comprises GSMparameters including timing information for effecting the handoff to theGSM system; generating a Mobile Application Protocol (MAP) messagecontaining the handoff data; and communicating the MAP message to thesecond base station.
 34. The method as claimed in claim 33, furthercomprising receiving from said second base station channel informationidentifying a channel in the second communications system for the mobilestation.
 35. The method as claimed in claim 34, further comprisingresponding to the channel information by establishing communicationbetween said mobile station and said second base station in theidentified channel.
 36. The method as claimed in claim 35, furthercomprising responding to the channel information by discontinuingcommunication between said mobile station and said first base station.37. The method as claimed in claim 33, wherein said first and secondparameters correspond to signal strength.
 38. The method as claimed inclaim 33, wherein said handoff data includes authentication data. 39.The method as claimed in claim 33, wherein the measuring of the firstparameter comprises activating a first transceiver chain at a time whena second transceiver chain corresponding to the measuring of the secondparameter is inactive.
 40. The method as claimed in claim 33, whereinthe measuring of the second parameter comprises activating a secondtransceiver chain at a time when a first transceiver chain correspondingto the measuring of the first parameter is inactive.